Diaryl carbonates, such as diphenyl carbonate, are an important reactant in the production of polycarbonate resins. As the uses to which polycarbonates are put have increased, the safe and efficient production of diaryl carbonates has become of greater significance. Early processes for the production of diaryl carbonates utilized phosgene as a reagent. The toxicity of phosgene, however, prompted the development of a non-phosgene process. The non-phosgene process is well-known in the art and is described in U.S. Pat. Nos. 4,410,464, 5,344,954, 6,784,277, 7,141,641, and 7,288,668, which are incorporated herein by reference.
As shown schematically in FIG. 1, the non-phosgene process involves three reaction steps. First, a dialkyl carbonate (e.g. dimethyl carbonate) and water are formed by reacting an alkanol (e.g. methanol) with oxygen and carbon monoxide in the presence of a catalyst. In a second reaction step, the dialkyl carbonate reacts with an aromatic alcohol (e.g. phenol) to produce an arylalkyl carbonate (e.g. phenylmethyl carbonate) and an alkanol (e.g. methanol). Then, in the third reaction step, two molecules of an arylalkyl carbonate undergo a transesterification reaction to produce one molecule of diaryl carbonate (e.g. diphenyl carbonate) and one molecule of dialkyl carbonate. The present invention relates to the first step of this process.
As described above, the formation reaction to form dialkyl carbonate (step 1 of FIG. 1) reacts an alkanol with carbon monoxide and oxygen in the presence of a catalyst usually in a closed stirred tank reactor. It has been found that catalyst can build up on horizontal surfaces in the reactor, in transfer lines to and from the reactor, and in separation equipment used later in the production process. This catalyst buildup has two potential problems. The first problem is that the catalyst buildup can, over time, clog transfer lines to and from the reactor. Furthermore, the catalyst buildup in the transfer lines as well as the horizontal surfaces in the reactor can lead to solid catalyst “chunks” breaking loose and falling into the reactor or later separation equipment. If the “chunks” are big enough they may interfere with and potentially damage the reactors agitation propellers or cause vibration and damaging conditions within separation equipment.
To avoid these events a dialkyl carbonate production facility would have to be shut down to clean the reactors, transfer lines, and separation equipment to remove any catalyst buildup that had occurred. The time the dialkyl carbonate production facility was shut down would create shortages to downstream consumers or processes that require a consistent stream of dialkyl carbonate. It would be extremely beneficial to find a process where catalyst buildup as described herein could be avoided and where a safe and equipment friendly process can produce a consistent stream of dialkyl carbonate.